The present invention relates to an aqueous dispersion suitable to applications such as primer and paint usable for painting polypropylene-based resin moldings and primer and adhesive for adhering polypropylene-based resin products with other substrates.
Based on excellent properties and low price, polypropylene-based resins are used for household electrical appliances, automobile parts, etc. in large quantities. However, polypropylene-based resins have drawback of difficulty in painting and adhesion because of their nonpolarity. In cases of painting and adhesion onto polypropylene-based resins, therefore, primer having chlorinated polypropylene as a major component is commonly used. However, since the chlorinated polypropylene is used by dissolving it into aromatic organic solvent such as toluene or xylene, large quantities of aromatic organic solvent are forced to be used, arising to problems in aspect of safety, sanitation and environmental pollution.
From this reason, attempts for producing aqueous dispersion of chlorinated polypropylene have been made. Aqueous dispersions of carboxyl group-containing chlorinated polypropylene (Japanese Unexamined Patent Publication No. Hei 1-256549, No. Hei 3-124779 and No. Hei 3-182534), aqueous dispersions of graft-modified chlorinated polypropylene (Japanese Unexamined Patent Publication No. Hei 1-256556, No. Hei 2-284973 and No. Hei 4-88026), mixture with other emulsions (Japanese Unexamined Patent Publication No. Hei 1-153778), use of reactive surfactant (Japanese Patent Application No. Hei 4-258936), and the like are proposed. These methods however employ organic solvent solution of chlorinated polypropylene as a starting material for dispersing into water and had a substantial drawback that large quantities of organic solvent were forced to be used in the process of water dispersion.
On the other hand, proposals for directly making aqueous dispersion from solid chlorinated polypropylene have also been made. Method of making aqueous dispersion through extruder after adding adhesive and poly(vinyl alcohol) to chlorinated polypropylene (Japanese Unexamined Patent Publication No. Sho 58-80337), method of adding ethylene-vinyl acetate copolymer and wax to chlorinated polypropylene followed by dispersing them into water with cationic or anionic acrylic copolymer (Japanese Unexamined Patent Publication No. Sho 58-120655 and No. Sho 59-4637), method of dispersing it into water with amino group-containing acrylic resin (Japanese Unexamined Patent Publication No. Hei 1-223147), and the like were proposed. These methods however had problematic point that many components other than chlorinated polypropylene were added which leads to poor adhesive property or inevitable use of biaxial extruder, then the initial investment thereof is increased. Moreover, a method of dispersing it into water with the aid of polyol (Japanese National Phase Publication No. Hei 4-506530) was proposed, which had a drawback of poor water resistance.
As described above, when making aqueous dispersion of chlorinated polypropylene, there were problematic points of using aromatic organic solvent such as toluene or xylene in large quantities in the production process thereof. This is because of that, since chlorinated polypropylene tends to decompose with dehydrochlorination under heat, it cannot help being dispersed into water at low temperature and hence the dispersion is difficult unless dissolving it into organic solvent. Moreover, the method of directly making aqueous dispersion by adding water in the molten state of solid chlorinated polypropylene had a problematic point that other components such as softener and modifier could not help being added, leading to poor performance.
As a resulting of diligent investigations to solve the problematic points aforementioned, the inventors have found that an aqueous dispersion with excellent adhesive property can be produced without using organic solvent, leading to the invention.